Thanks Doug and Bruce Does anyone have an opinion regarding the necessity of scanning the ADNI phantom?
Best wishes Heath On Wed, Sep 12, 2012 at 1:03 PM, Douglas N Greve <gr...@nmr.mgh.harvard.edu>wrote: > If you include scanner as an interaction nuisance variable and balance > your design so that there are an equal number of controls and patients > for each scanner, you're probably ok. A design where you have all the > controls from one scanner and all the patients from another is probably > a bad idea. > doug > > On 09/12/2012 08:43 AM, Bruce Fischl wrote: > > Hi Heath > > > > yes, I think it's a bad idea for thickness just as it would be for > > VBM. ADNI is a bit of a special case as they went to great lengths to > > make their acquisitions uniform across scanner platform, but I would > > still look to see if there is a significant scanner effect in > > analyzing that data. > > > > cheers > > Bruce > > > > > > > > On Tue, 11 Sep 2012, Heath Pardoe wrote: > > > >> Hi all, > >> I was wondering if there was a consensus view on the optimal way to > >> approach morphometric analysis (cortical thickness, > >> volumetry & associated analyses) of MRI data acquired on multiple > >> scanners. > >> > >> From VBM literature & our own experience it appears that comparing > >> MRI scans of patients from one scanner with controls scanned > >> on another is a bad idea, because there are systematic differences > >> that will manifest as very significant GM > >> volume/concentration/density differences following statistical > >> analysis when these differences are unlikely to really exist ie. > >> we get a lot of false positives. > >> > >> Definitive findings on similar analyses using cortical thickness > >> mapping seem harder to come by. There's the Han study from 2006 > >> and Dickerson 2008 but both of these seem to be empirical analyses of > >> mean absolute error or related measures rather than > >> pitting scanner 1 subjects against scanner 2 subjects in a > >> statistical analysis (please correct me if I haven't looked hard > >> enough). Wonderlick et al 2009 is closer in terms of statistical > >> analysis but these were done on one scanner so don't really > >> address the multicenter issue. > >> > >> To my mind it seems likely that cortical thickness analysis would > >> have the same problem as VBM and would require controls to be > >> scanned on the same scanners as patients to account for inter-scanner > >> variability. Does anyone have any opinions on this? Or, > >> even better, any data so we can tell how much of a problem this is? > >> > >> In a related question, the ADNI study uses a phantom to correct an > >> image for gradient nonlinearities in different scanners, and > >> then pools this corrected data from multiple scanners into each > >> control or patient group, and subsequent morphometric analyses > >> are done without worrying about the multi-site issue. Does anyone > >> have an opinion on the validity of this approach? > >> > >> I guess one way to investigate it would be to look at vertex-wise > >> across-subject variance in coregistered cortical thickness > >> maps both with and without the phantom-based correction procedure; if > >> the variance was lower in the phantom-corrected variance > >> map this would provide good evidence that it's worth the effort of > >> undertaking this procedure. I would be keen to do this > >> analysis but unless I'm mistaken the ADNI website only provides > >> images that have already been processed to correct for gradient > >> nonlinearities. > >> > >> An alternative approach that has been proposed is to scan a group of > >> the same controls on multiple scanners to characterise > >> between-scanner differences. The practicality of this approach breaks > >> down as the number of centers increases. Again I would be > >> interested to hear if anyone has experience in using this approach > >> compared to others. > >> > >> In summary, there seem to be three ways to overcome inter-scanner > >> variance: > >> 1. scan different controls at each site > >> 2. scan a phantom at each site (and probably scan controls too anyway) > >> 3. scan the same group of controls at all sites > >> > >> I would be interested in hearing which approach people think is best, > >> and why. > >> > >> Thanks if you're still reading! > >> Best wishes > >> Heath > >> > >> > > > > > > _______________________________________________ > > Freesurfer mailing list > > Freesurfer@nmr.mgh.harvard.edu > > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > > -- > Douglas N. Greve, Ph.D. > MGH-NMR Center > gr...@nmr.mgh.harvard.edu > Phone Number: 617-724-2358 > Fax: 617-726-7422 > > Bugs: surfer.nmr.mgh.harvard.edu/fswiki/BugReporting > FileDrop: www.nmr.mgh.harvard.edu/facility/filedrop/index.html > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer > > > The information in this e-mail is intended only for the person to whom it > is > addressed. If you believe this e-mail was sent to you in error and the > e-mail > contains patient information, please contact the Partners Compliance > HelpLine at > http://www.partners.org/complianceline . If the e-mail was sent to you in > error > but does not contain patient information, please contact the sender and > properly > dispose of the e-mail. > > -- Heath R. Pardoe, PhD Florey Neuroscience Institutes Melbourne Brain Centre 245 Burgundy Street, Heidelberg 3084 Australia Phone: (+61 3) 903 57053 Email: h.par...@brain.org.au
_______________________________________________ Freesurfer mailing list Freesurfer@nmr.mgh.harvard.edu https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer The information in this e-mail is intended only for the person to whom it is addressed. If you believe this e-mail was sent to you in error and the e-mail contains patient information, please contact the Partners Compliance HelpLine at http://www.partners.org/complianceline . If the e-mail was sent to you in error but does not contain patient information, please contact the sender and properly dispose of the e-mail.
